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List:       sip-implementors
Subject:    Re: [Sip-implementors] Multiple early media sessions within	a	samedialog
From:       Paul Kyzivat <pkyzivat () cisco ! com>
Date:       2011-01-08 21:30:58
Message-ID: 4D28D792.4050000 () cisco ! com
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You are right that in theory the announcement can be sent without 
sending an answer. But in practice its often impossible to get media 
received without sending an answer first. Either the UAC will restrict, 
or in some cases an SP will gate media based on the answer.

	Thanks,
	Paul

On 1/8/2011 9:08 AM, Kevin P. Fleming wrote:
> On 01/07/2011 09:47 PM, Paul Kyzivat wrote:
>>
>>
>> On 1/7/2011 9:21 PM, SIP Satan wrote:
>>> Cant we play multiple announcements by giving different SDP's  in
>>> multiple 1xx responses provided
>>> each 1xx carries a different To-tag. In a way simulating forking
>>> environment.
>>
>> Yeah, should be fine. To the other end its indistinguishable from "real"
>> forking.
>>
>> That of course assumes that the UAC is capable of rendering media from
>> different early dialogs.
>
> Maybe I'm missing something obvious (it is Saturday morning after all),
> but why does the UAS need to send an SDP *at all* in order to play
> announcements? If the UAC included an SDP offer in the initial INVITE,
> then it is prepared to receive media on any sessions in that offer that
> are either 'active' or 'recvonly', whether it receives an answer or not.
> If the UAC did not include an SDP offer in the initial INVITE, then
> announcements can't be played until the UAS sends a 200 OK (with SDP)
> and receives an ACK (with SDP).
>
> Since announcements are typically unidirectional media (the announcement
> server doesn't need to receive media from the caller, and would just
> throw it away if it did), then the media can be sent to the caller
> without doing anything more than sending a single '183 Session
> Progress', and it does not even need to include an SDP answer.
>

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